Our depth sight is connected with the fact that the right and left eyes see the surroundings from a different place and under a somewhat different angle. One eye sees a picture differing from the picture that the other eye sees, and the brain co-ordinates the two such that we experience three dimensions.
It is known to create pictures with a three-dimensional effect by letting each right and left eye see a picture of its own, e.g. two photos taken from two points spaced correspondingly to a normal mutual distance between the eyes of a human being. There exist special cameras for such purposes, so-called stereo cameras, having two objectives.
In recent years, techniques used in order to achieve three-dimensional effect in photos, developed to comprise pictures that can be transferred electronically, such as video and digitized images, and a technique has been developed that makes it possible to show both still pictures and moving pictures on screen.
To let a viewer experience three-dimensional effect, the picture photographed or made in some other way for the right eye must be shown for the right eye, and the picture photographed or made in some other way for the left eye, must be shown for the left eye. If both pictures are shown for both eyes, a blurred (unsharp) picture is experienced, and the three-dimensional effect fails.
In order to avoid the right eye seeing the picture belonging to left eye, and vice versa, the pictures may be viewed through an ocular for each eye, in a so-called stereoscope. This gives a good three-dimensional effect, but it is not very suited for pictures to be viewed by several persons simultaneously, e.g. in a cinema hall.
It is known to divide a right and left picture in narrow stripes which are assembled alternately to form one picture. When viewing the picture stripes through glass or plastic, where prisms are formed parallel to the pictures stripes, the result is that the right eye sees picture stripes belonging to right picture, and that the left eye sees picture stripes belonging to left picture.
Further, it is known to print two pictures, one for the right eye and one for the left eye in registry on paper. Such pictures are viewed through special spectacles separating the pictures from each other, so that the right eye sees one picture and the left eye sees the other picture.
In one type of spectacles, spectacle glasses having different color for each eye are used, such as a red and a bluish green. Each picture is prefiltered before printing. The right picture is printed in complementary color to the left picture and left spectacle glass, and vice versa. Then, each eye sees a different picture. The technique is also used when projecting two pictures in registry on a screen (canvas), and it is possible to show moving pictures, film and animations in this way.
The technique which also may be used for television, has several disadvantages. The filtration and the spectacle glasses influence the color balance, and an adequate separation of the pictures for right and left eye is not achieved. Each eye experiences a portion of the picture meant for the opposite eye and the picture is, thus, experienced as unsharp.
Another known way of separating pictures for the right and left eyes consists in that a picture for each eye is projected in registry on a screen by means of polarized light. The polarization for the one picture is at right angles to the polarization for the other, and the viewer uses spectacles having glasses each correspondingly being polarized for letting through light for one of the pictures only. By means of this, less color error is achieved than by using a color filter, and a better picture separation is obtained.
Upon transfer of electronic pictures, such as video pictures, it has been found to be difficult to synchronize two parallel picture signals in a flickerfree way. In connection with projecting video pictures or pictures from computers, it has proved advantageous to transfer picture for alternate right and left eye in a common channel in lieu of in two parallel channels. This means that every second picture transferred, belongs to right eye, while the rest belongs to left eye. The pictures are projected on a screen and are viewed through spectacles having glasses which can shut and open in step with an electric signal alternating synchronously with the pictures. Such spectacle glasses utilize liquid crystals. The left spectacle glass is shut while the right picture is projected, and the right spectacle glass is shut while the left picture is projected.
This known technique gives a good effect, but it has several disadvantages. The spectacles are expensive, and they have to be provided with an electric signal for synchronization with the stream of pictures, which can be difficult in a cinema hall. In practice, the technique is usable only for stationary plants. Also, great demands are made upon the projector which has to operate with double picture frequency. The high picture rate means that reasonable projectors in which the picture is formed by liquid crystals, can not be used.